STAT3 and neoplasm: Moreover, the recruitment of myeloid-derived suppressor cells (MDSC) and tumor-associated macrophages (TAM) into the TME by CSCs and their constant interaction, contribute not only to the establishment of an immunosuppressive TME and increased expression of PD-1 and PD-L1 by T lymphocytes and CSCs, respectively, but also promotes CSCs maintenance and survival via different mechanisms, including activation of mTOR, NF-κB, STAT3, and Src signaling pathways and secretion of different cytokines [12–14].